The landscape and turf industries are based on beauty. Some trees in the landscape
suppress the growth of any other plant species beneath them. This is reported to be
caused by the presence of allelochemicals which are released into the soil by the plants, a
mechanism known as allelopathy. Soil amendment is therefore needed to curb the effects
of these allelochemicals and make the nutrients in the soil available to other plant species
that may be planted beneath the allelopathic trees. There is evidence from thousands of
years of traditional use of charcoal as amendment in the terra preta soils of Brazil.
Biochar, a pyrolised biomass, is a fine-grained, highly porous charcoal substance that is
used as a soil amendment. The study was conducted to find out the ameliorative effect of
biochar and charcoal in allelochemical-laden soils to improve on the physicochemical
properties of the soil. Charcoal produced from Tectona grandis tree, biochar produced
from four different types of sawdust (Tectona grandis, Celtis mildbraedii,
Entandrophragma cylindricum and Khaya senegalensis) and absolute control were the
treatments used. The study was carried out beneath three trees suspected to be
allelopathic (Tectona grandis, Eucalyptus grandis and Bambusa sp.). St. Augustine’s
grass was used for the study because it prefers shaded growing environmental conditions.
The experimental design employed was Randomized Complete Block Design (RCBD)
and the experiment was replicated three times. Data collected over a period of twelve
weeks included presence of allelochemicals in the soil and in the tree species, rate of
spread and rate of growth of grass, weed count, soil nutrient analysis, soil water-holding
capacity, presence of soil microorganisms and soil pH. The results of the study indicated
that the three landscape trees are allelopathic. The biochar and charcoal were able to
ameliorate the effects of the allelochemicals and hence allowed the grass to grow well
under the trees. Significant differences were observed in the rate of spread of grass as
well as the rate of growth. Available phosphorous and soil potassium were increased in
the biochar amended plots whereas total nitrogen was reduced due to adsorption of NH
3-and NH
4+
from the soil solution onto the biochar surface. Soil organic carbon was
reduced due to the priming effect of biochar. Water-holding capacity was increased
tremendously in all amended plots due to the porous nature of the charcoal and biochar.
Both amendments were able to improve soil pH to an optimum range for most plants. It
was recommended that other feedstock types for biochar and charcoal production should
be considered in further studies to find out which feedstock type can also improve the soil
where allelochemicals have been found.

Description:

A thesis submitted to the school of research and graduate
Studies, Kwame Nkrumah University of Science and
Technology, in partial fulfillment of the requirements
for the award of Master of philosophy (Mphil landscape
Studies) degree, 2013